Organ formation in plants occurs via the activity of apical meristems. Plant meristems contain a pool of stem cells, which are able to self-maintain, give rise to a variety of cell types including cells required for organ initiation. The initiation and maintenance of stem cells and their integration into organ-forming meristems are thus the basis for continuous plant development.
The Wuschel protein, designated hereafter as WUS, plays a key role in the initiation and maintenance of the apical meristem, which contains a pool of pluripotent stem cells (Endrizzi et al., 1996, Plant Journal 10:967 979; Laux et al., 1996, Development 122:87 96; and Mayer et al., 1998, Cell 95:805 815). Arabidopsis plants mutant for the WUS gene contain stem cells that are misspecified and that appear to undergo differentiation. WUS encodes a novel homeodomain protein, which presumably functions as a transcriptional regulator (Mayer et al., 1998, Cell 95:805 815). The stem cell population of Arabidopsis shoot meristems is believed to be maintained by a regulatory loop between the CLAVATA (CLV) genes which promote organ initiation and the WUS gene which is required for stem cell identity, with the CLV genes repressing WUS at the transcript level, and WUS expression being sufficient to induce meristem cell identity and the expression of the stem cell marker CLV3 (Brand et al. (2000) Science 289:617-619; Schoof et al. (2000) Cell 100:635-644). Constitutive expression of WUS in Arabidopsis has been recently shown to lead to adventitious shoot proliferation from leaves (in planta) (Laux, T., Talk Presented at the XVI International Botanical Congress Meeting, Aug. 17, 1999, St. Louis, Mo.).
There is a great deal of interest in identifying the genes that encode proteins involved in development in plants, generally toward the objective of altering plant growth and architecture. WUS represents one such gene. However, the WUS gene can also be used for the novel application of stimulating in vitro growth of plant tissue and improving transformation. In this manner, this gene can expand the range of tissues types targeted for transformation. Specifically, the WUS gene may be used to improve meristem transformation frequencies and could result in genotype independent transformation of many important crops such as maize, soybean and sunflower. Furthermore, transformation into meristems would stimulate the formation of new apical initials reducing the chimeric nature of the transgenic events. Lastly, ectopic expression into non-meristematic cells would stimulate adventive meristem formation. This could lead to transformation of non-traditional tissues such as leaves, leaf bases, stem tissue, etc. Alternatively, transformation of a more traditional target such as callus or the scutellum of immature embryos could promote a “non-traditional” growth response, i.e. meristems in place of somatic embryos. In addition, WUS may also be used as a genetic marker for meristems. Accordingly, the availability of nucleic acid sequences encoding all or a portion of a WUS protein would facilitate studies to better understand programmed development in plants, provide genetic tools to enhance the efficiency of gene transfer into meristem tissue and help provide alternative transformation methods in several important crops.